Eighteen months ago, Chris Larner took his ex-wife to Switzerland. She was in the later stages of multiple sclerosis: her limbs barely functional, exhausted by just 15 minutes conversation. She had asked him, when the time was right, to help her to die; a year later, they were on their way to Dignitas, to face the end together.

On a bare stage, Larner shares his tale – of Allyson's life, of their journey, and of the illness that brought her to it – in just over an hour. Each minute is transfixing, because it focuses not on the moral dilemma of assisted suicide (although Larner has his moments of doubt) but on the intimate details – the preparation, the paperwork, the tragicomedy of keeping their mission secret from the myriad social services. And on Allyson herself: tough-minded Lancastrian, inspirational teacher and mother, whose son doesn't want her to leave.

Larner is a consummate storyteller, engaging his audience to such a degree that I heard people murmur their assent as they recognised places or identified with feelings. Regular infusions of laughter – not just gallows humour but genuine levity – keep the experience from becoming too grim to bear (Allyson decides not to floss on the day she dies: "Today I can stop worrying about dental decay"). But don't kid yourself: you'll need a hanky. This isn't a triumphal advert for Dignitas but a portrait of the desperation that drives people there.